In the past, there has been proposed an infrared sensor which detects infrared (e.g., infrared having a wavelength of 8 to 12 μm which emitted from a human body). A document 1 (Japanese patent publication No. 2576259) and a document 2 (Japanese patent publication No. 3287173) disclose infrared sensors manufactured by use of micromachining techniques. This kind of infrared sensor includes a thin film-shaped infrared absorption member and a temperature detection member. The infrared absorption member absorbs infrared and converts the absorbed infrared into heat. The temperature detection member measures a change in temperature of the infrared absorption member.
Wherein, the infrared sensor disclosed in the above document 1 includes a silicon substrate, and a silicon nitride film formed on the silicon substrate. The silicon substrate is provided with a cavity for thermal insulation. The silicon nitride film has its portion covering the cavity which acts as the infrared absorption member. In this infrared sensor, a thermopile is adopted as the temperature detection member. The thermopile is made of an n-type silicon film and a p-type silicon film formed on the silicon nitride film, by use of patterning techniques and the like.
The infrared sensor disclosed in the document 2 includes a dielectric layer (infrared absorption member) having high absorption in infrared wavelength region. On the dielectric layer is formed a temperature detection semiconductor layer. Below the dielectric layer under the temperature detection semiconductor layer is formed a cavity for suppressing conduction of heat from the temperature detection semiconductor layer to an outside. In this infrared sensor, the dielectric layer is preferred to be thinned in order to decrease a heat conductance thereof (increase a resistance thereof) and in order to increase a response speed. When the dielectric layer is thinned, the dielectric layer being the infrared absorption member is likely to suffer from a warp, and the infrared sensor may have poor structural stability and low sensitivity.
In addition, the infrared sensor disclosed in the document 2 uses a bolometer type infrared detection element. Therefore, it is necessary to flow a current in order to measure a change in a resistance of the infrared detection element. This causes increased power consumption. Moreover, since the bolometer type infrared detection element heats itself, the infrared detection element is likely to suffer from a warp caused by thermal stress resulting from the self-heating. Further, a resistance temperature coefficient of the infrared detection element is changed by a temperature variation resulting from the self-heating and/or a surrounding temperature variation. In view of this, a temperature compensation polysilicon layer is necessary to improve accuracy. However, to provide the temperature compensation polysilicon layer enlarges the infrared sensor, and increases a production cost.
Meanwhile, in the infrared sensor disclosed in the document 1, the thermopile is used as the temperature detection member. Differently from the bolometer type infrared detection element, the thermopile needs no current to measure the temperature. Thus, the thermopile does not produce self-heating. Therefore, the infrared absorption member is kept free from a warp caused by the self-heating. Further, power consumption can be reduced. In addition, since a sensitivity of the thermopile does not depend on its temperature, the thermopile has high accuracy.
In order to form the thermopile disclosed in the document 1, it is necessary to pattern, by use of etching techniques, the n-type silicon film and the p-type silicon film formed on the infrared absorption member. When the thermopile is formed, the infrared absorption member is likely to be etched together with the p-type silicon film and/or the n-type silicon film. In this situation, a thin film structure comprising the infrared sensor and the thermopile formed thereon is likely to suffer from a warp, and the infrared sensor may have poor structural stability and low sensitivity.